Clutches are rotational power transmission apparatus which cause a driven load to be rotated by a driver when engaged and allow the load to freewheel when disengaged. A clutch typically includes a shaft-mounted portion which is securely affixed to a shaft and which includes at least one movable clutch plate, a means of moving the clutch plate, and a piloted portion which is rotatably piloted to the shaft-mounted portion by bearings and precision fits and includes a member having a friction surface to be engaged by the clutch plate. Clutches similarly configured and having a plurality of clutch plates and friction plates are commonly used in automatic and powershift transmissions for on-highway and off-road vehicles as well.
Methods of moving clutch plates in and friction plates into and out of engagement with each other vary, with one most common method being use of hydraulic pressure of a fluid to move a piston which moves the clutch plate and the friction plate into engagement with each other.
Typically, the clutch plate and the friction plate are surrounded by a housing which forms a chamber. Lubricating fluid within the chamber lubricates the plates, bearings, and seals, and a continuous flow of the fluid carries off heat. When the clutch is engaged, the clutch plate and friction plate are not slipping with respect to each other and therefore generate no heat. However, a small amount of heat may be generated by parasitic loads such as bearing windage, lubricant churning, and seal drag. When the clutch is disengaged, some relatively minor and intermittent drag of clutch plate upon friction plate, either dry or caused by viscous shear of residual fluid, may occur which will add to the above parasitic loads. Normal lubrication and cooling needs are served while the clutch is engaged or disengaged by a low flow rate of lubricant into the chamber.
During the time that the clutch is being engaged, however, a large amount of heat is generated by slipping of the clutch plate with respect to the friction plate until one of them brings the other up to speed, particularly if a large torque or inertial load is coupled to the member being brought up to speed. To prevent degradation of the clutch plate and friction plate materials and other thermal damage to the clutch, it is necessary to remove this large amount of heat generated during the process of engagement/disengagement. This can be done by correspondingly increasing the fluid flow rate. It is not economically or technically desirable, however, to maintain the high rate of fluid flow during the relatively long periods of clutch steady state operation in disengaged or engaged conditions.
It is known to temporarily either begin or increase a flow rate of fluid during engagement of a clutch in order to dissipate the heat generated by engagement, and to either stop or reduce the flow of fluid during operation either engaged or disengaged. Some innovations have used relatively complex control systems to control fluid flow (e.g., U.S. Pat. No. 4,540,078, Wetrich, 1985; U.S. Pat. No. 4,640,401, Koltookian, 1987). Such control equipment, however, adds otherwise unnecessary expense and complexity to an otherwise simple apparatus. Others have used partial covering and uncovering of transversely oriented ports in a shaft by sliding movement of clutch parts (e.g., U.S. Pat. No. 5,577,588, Raszkowski, 1996; U.S. Pat. No. 4,529,073, Lewis, 1985; U.S. Pat. No. 4,544,055, Kronstadt, 1985; U.S. Pat. No. 4,270,647, Leber, 1981). Such devices are generally sleeve valves of a size large enough to require tightly held machining tolerances to avoid copious leakage, and are not able to compensate for repositioning due to wear of clutch plates and/or friction surfaces. Others have utilized a valve member installed in a passage formed within a shaft (e.g., U.S. Pat. No. 5,577,588, Raszkowski, 1996; U.S. Pat. No. 3,301,367, Yokel, 1967; Japanese Pat. No. 04019426A, 1992, Hirano et al.; U.S. Pat. No. 5,613,588, Vu, 1997), but it is generally preferable to include all parts of a clutch within the clutch assembly and to not have separate parts, such as valve members, which must be separately installed to a shaft.
It would be advantageous to provide for a clutch which increases flow of a cooling and lubricating fluid when engagement of the clutch creates heat, without need of complex external controls or separate valving members. It would also be advantageous to provide for such a clutch which does not utilize valving members which require stringent machining tolerances, wherein the performance of which may change when the clutch becomes worn.